Boron/nitrogen preceramic polymers and boron nitride ceramic materials produced therefrom

ABSTRACT

Crosslinked boron/nitrogen preceramic polymers are prepared by reacting (a) admixture of a trihalogenoborane A and a compound B containing at least one dihalogenated boron atom, with (b) an amino compound C containing at least one -NH2 group; the resulting polymers are facilely pyrolyzed into boron nitride ceramic materials.

CROSS-REFERENCE TO COMPANION APPLICATIONS

Our copending applications, Ser. No. 242,977 and Ser. No. 243,827, bothfiled concurrently herewith and both assigned to the assignee hereof.

BACKGROUND OF THE INVENTION

1. Field of the Invention:

The present invention relates to novel polymers based on boron andnitrogen and to a process for the preparation thereof.

This invention also relates to the use of such novel polymers in theproduction of ceramic materials and shaped articles based on boronnitride, especially boron nitride in fibrous form.

2. Description of the Prior Art:

Boron nitride is increasingly in demand in this art in light of its highthermal stability, its resistance to thermal shock, its great chemicalinertness and its very good thermal conductivity. On the other hand, itslow electrical conductivity makes it an insulator of choice.

Several processes are presently known to the art for the preparation ofboron nitride.

One such process includes reacting boron trichloride with ammonia in thegaseous state. A fine boron nitride powder is obtained in this manner,which may be sintered to produce solid shaped articles. However, theshaped articles thus produced exhibit a characteristic microporositywhich may be highly detrimental for certain applications.

More recently, it was discovered that boron nitride could be produced bythe pyrolysis of precursor polymers.

The advantage of this "polymer" method primarily resides in the form ofthe final product, and, more particularly, enables the production, afterpyrolysis, of boron nitride fibers.

Thus, in U.S. Pat. No. 4,581,468 a preceramic organoboron polymer isdescribed which is prepared by the interaction of ammonia (ammonolysis)with B-trichloro-N-tris(trialkylsilyl)borazine (a cyclic compound) andwhich, as indicated, after drawing and pyrolysis at 970° C, results inthe production of boron nitride fibers.

However, the cyclic polymer described in this patent is very difficultto prepare and is thus expensive. Therefore, it is not suitable forproduction on an industrial scale.

On the other hand, the maximum yield by weight of boron nitride that canbe produced from such type of starting material does not exceed 22%,indicating that actual yields are on average well below this value.

SUMMARY OF THE INVENTION

Accordingly, a major object of the present invention is the provision ofa simple, efficient, economical and readily applicable improved processfor the preparation of organometallic polymers based on boron andnitrogen in a wide variety of useful forms (wires, fibers, molded shapedarticles, coatings, foils, films, and the like), and which various formsare facilely converted in high yields by weight, upon pyrolysis, intouseful materials based on boron nitride.

Briefly, it has now surprisingly and unexpectedly been determined thatboron nitride can be prepared, in high yields by weight, from precursorpolymers based on boron and nitrogen, by reacting (a) a mixturecontaining a trihalogenoborane (compound A) and a boron compound whichcomprises at least one boron atom to which two halogen atoms aredirectly bonded (compound B), with (b) a compound containing at leastone NH₂ group (compound C).

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

More particularly according to the present invention, in respect of theaforesaid compounds A and B a functionality f is assigned which is equalto the number of halogen atoms directly bonded to the boron atom. Thus,compound A, which is a trihalogenoborane, has a functionality of 3,while compound B has a functionality of 2.

The compounds C will hereinafter be designated aminolysis reagentsgenerally (an amine compound having at least one NH₂ group), and anammonolysis reagent in the more particular case where ammonia is used.

Also, an in consequence of the above, the reaction products producedfrom the compounds A, B and C shall hereinafter be designated, dependingon the particular case, aminolysates or ammonolysates, with the latterof course being included in the generic class of "aminolysates".

These aminolysates, as more fully explained below, constitute the novelpolymers based on boron and nitrogen, which circumscribe another objectof the present invention.

The preparative process according to this invention essentiallycomprises a co-aminolysis of a mixture of at least one compoundcontaining a difunctional boron atom and at least one compoundcontaining a trifunctional boron atom.

Thus, it has surprisingly and unexpectedly now been discovered that suchco-aminolysis enables the production of polymers having a structureconstituting a particularly cross-linked network which imparts greatthermal stability during pyrolysis, thereby increasing the yield inboron nitride.

The starting trifunctional compound A is advantageously trichloroborane,although other halogenoboranes are also suitable, such as, for example,a trifluoro-, a tribromo- or a triiodoborane.

The starting difunctional compound B is advantageously also a chlorinecompound.

This compound preferably has the following general formula (1): ##STR1##wherein Y represents: ##STR2## in which R¹ and R², which may beidentical or different, are each a hydrogen atom, or a hydrocarbyl,organosilyl or hydrogenoorganosilyl radical.

The hydrocarbyl radicals are advantageously alkyl, cycloalkyl, aryl,alkylaryl or arylalkyl radicals, as well as alkenyl or alkynyl radicals.

Representative such alkyl radicals include the methyl, ethyl, propyl,butyl, pentyl, hexyl, heptyl and octyl radicals. Representativecycloalkyl radicals include the cyclopentyl, cyclohexyl and cycloheptylradicals. Exemplary aryl radicals are the phenyl and naphthyl radicals,and exemplary alkylaryl radicals are the tolyl and xyxyl radicals.Representative arylalkyl radicals are the benzyl and phenylethylradicals.

Illustrative of the alkenyl radicals are the vinyl, allyl, butenyl andpentenyl radicals, and illustrative alkynyl radicals are the ethynyl,propynyl and butylyl radicals.

In a preferred embodiment of the invention, the radicals R¹ and R² areorganosilyl or hydrogenoorganosilyl radicals, and more particularly(triorgano)silyl or (hydrogenodiorgano)silyl radicals. Even morepreferably, (trialkyl)silyl radicals are used, such as, in particular,trimethyl-, triethyl-, tripropyl-, tributyl-, tripentyl-, trihexyl-,triheptyl- or trioctylsilyl radicals. Trimethyl)silyl radicals areespecially preferred.

The compounds of Formula (1) are well known to this art and may beprepared by any method itself known to the art.

For example, in the case of alkyl type R¹ and R² radicals, see Wilbergand Schuster (Zeitschrift Fur Anorganische Chemie, 213, page 77 (1933)),Brown (Journal of the American Chemical Society, 74, page 1219 (1952)),or Burg and Banus (Journal of the American Chemical Society, 76, page3903 (1954)).

Concerning the radicals R¹ and R² of triorganosilyl type, see Jenne andNiedenzu (Inorganic Chemistry, 3, 68 (1964)), Sujishii and Witz (Journalof the American Ceramic Society, 79, page 2447 (1957)), or Wannagat(Angew. Chemie, International Edition, 3, page 633 (1964)),

In general, the difunctional compound B may be prepared by the action ofBCl₃ on ##STR3## under suitable conditions of temperature and molarratio.

Finally, concerning the aminolysis reagents (compound C) according tothe invention, ammonia, the primary amines, the diamines (hydrazine,alkylhydrazine, hydrazide, alkylenediamines, etc.), the amides, thesilylamines, and the like, are exemplary.

However, preferably the compounds having the following general formula(2) are used: ##STR4## wherein the radical R³ is a hydrogen atom, or ahydrocarbyl, organosilyl or hydrogenoorganosilyl radical. The followingare particularly representative:

(i) ammonia (R³ =hydrogen atom);

(ii) the primary organoamines (R³ =alkyl, cycloalkyl, aryl, alkylaryl orarylalkyl), such as, for example, methylamine, ethylamine, propylamine,butylamine, pentylamine, hepthlamine and octylamine, cyclopropylamine,phenylamine, and the like;

(iii) silylamines and more particularly triorganosilylamines, such as(trimethylsilyl)amine and (triethylsilyl)amine, or thehydrogenodiorganosilylamines, such as, for example,(hydrogenodimethylsilyl)amine.

The preferred aminolysis reagents are the primary alkylamines andammonia.

In a more preferred embodiment of the invention, ammonia is used.

The general reaction scheme of the aminolysis in the reaction medium isas follows: ##STR5##

The aminolysis reaction may be carried out in mass or, preferably, in anorganic solvent medium (hexane, pentane, toluene, etc.), under anhydrousconditions.

The operation is typically carried out under atmospheric pressure,although lower or higher pressures are also within the ambit of thisinvention.

On the other hand, the aminolysis reactions are characteristicallyrather exothermic and it is thus preferred to operate at a lowtemperature.

The duration of the reaction, as a function of the amounts of thereagents introduced, may range from a few minutes to several hours.

The molar ratio in the initial mixture between the trifunctionalcompound (form T) and the difunctional compound (form D) may vary to avery large degree. In general, it is observed that the higher thepercentage of the trifunctional compound in the mixture, the higher willbe the pyrolytic yield in boron nitride of the polymer produced uponcompletion of the reaction. In a preferred embodiment of the invention,the molar ration T/D in the initial mixture is at least 1.

At the end of this reaction stage, the polymer is separated from thereaction medium, in particular the ammonium chlorhydrate formed, by anyknown means, for example by filtration or extraction and decantation bymeans, in particular, of an ammonia solution.

The polymer recovered in this manner, optionally after the eliminationof the solvent and drying, constitutes the production.

In addition to the aforedescribed preparative process, the inventionalso features novel polymers based on boron and nitrogen producedthereby, and which, after pyrolysis thereof, result in the production ofhigh yields by weight of boron nitride.

Thus, it has now also unexpectedly been determined that high weightyields of boron nitride can be produced from a polymer precursor basedon boron and nitrogen, said precursor polymer comprising, per molecule:

(a) at least one structural unit of the formula (I): ##STR6## and (b) atleast one structural unit of the Formula (II): ##STR7## wherein Yrepresents N-R¹ R² and X represents N-R³, in which R¹, R² and R³, whichmay be identical or different, are each a hydrogen atom, or ahydrocarbyl, organosilyl or hydrogenoorganosilyl radical.

It has now been discovered that such polymers based on boron andnitrogen, which have a network structure essentially comprising acombination of units of Formula (I) and units of Formula (II) such asdefined above, have a significantly improved resistance to pyrolysisrelative to the known precursors. Consequently, it is thus possible toproduce ceramic materials based on boron nitride in good yields byweight.

It will be appreciated that the polymers comprising all of the desired(I) and (II) units, i.e., all of the desired R¹, R² and R³ radicals, canbe prepared simply by reacting the compounds B and C defined above,which contain the same R¹, R² and R³ radicals.

By extrapolating, relative to the novel polymers of the presentinvention, the aforesaid concept of functionality as regards thecompounds A and B, the structural units of Formula (I) may formally beconsidered as being trifunctional, as they result from the aninolysis ofa trifunctional boron compound starting material (compound A), and thestructural units of Formula (II) as being difunctional, as they resultfrom the aminolysis of a difunctional boron compound starting material(compound B).

Similarly, the ration between the units of Formula (I) and those ofFormula (II) in the final polymer may be adjusted in a simple manner byutilizing an appropriate ratio between the compounds A and compounds Binitially present in the reaction medium.

In a preferred embodiment of the invention, the polymer contains atleast 50 molar % of units of Formula (I).

In general, it is observed that the higher the percentage of units (I),the higher the yield in boron nitride after pyrolysis.

As regards the hydrocarbyl and organosilyl radicals suitable for thepolymers according to the invention, refer to the different examplesgiven above of the identical R¹, R² and R³ of compounds B and C.

Thus, alkyl, cycloalkyl, aryl, alkylaryl and arylalkyl radicals, and(triorgano)sily radicals, such as, for example, the (trialkyl)silylradicals, are especially suitable.

In another preferred embodiment of the invention, the R³ radical isselected from among a hydrogen atom and the alkyl radicals.

Even more preferably, the R³ radical is a hydrogen atom.

On the other hand, in order to obtain the best yields in ceramics in thepyrolysis, it is preferable to select the R¹ and R² radicals from amongthe organosilyl radicals, and, more particularly, from among the(trialkyl)silyl radicals.

In a particularly preferred example of a polymer according to theinvention, the units (I) and (II) are of the following type: ##STR8##

The polymers according to the invention have a number average molecularweight ranging from 300 to 50,000, preferably from 500 to 5,000.

They also have a weight average molecular weight of from 600 to 100,000,preferably from 1,000 to 10,000.

Depending on the molar ratio existing between the units of Formula (I)and those of Formula (II), the polymers according to the invention maybe present, at ambient temperature, in a form varying from a ratherviscous or highly viscous oil to the solid state. In general, a highproportion of units of Formula (I) corresponds to a high molecularweight polymer and thus to a high viscosity.

The polymers according to the invention are soluble in most of the usualorganic solvents (hexane, toluene, and the like), which may be quiteadvantageous for the shaping thereof.

The polymers based on boron and nitrogen according to the invention areespecially useful in the manufacture of ceramic materials and shapedarticles at least in part comprising boron nitride.

In the most general case (the production of ceramic powders), thepolymer is pyrolyzed in an inert atmosphere, under vacuum, or preferablyin ammonia, at a temperature of from 100° to 2,000°C., until the polymeris entirely converted into boron nitride.

The polymer may be formed prior to pyrolysis, by molding or drawing, forexample. If it is desired to produce fibers, the polymer is drawn bymeans of a conventional drawing die (possibly after melting, if thepolymer is initially in the solid state), then heat treated at atemperature of from 100° to 2,000° C. and preferably under an ammoniaatmosphere, to yield boron nitride fibers.

The resulting fibers may then be used, e.g., as reinforcing materialsfor composites of the ceramic/ceramic or ceramic/metal type.

In order to further illustrate the present invention and the advantagesthereof, the following specific examples are given, it being understoodthat same are intended only as illustrative and in nowise limitative.

EXAMPLE 1:

This example illustrates the ammonolysis of a difunctional boroncompound, and therefore is outside of the scope of the presentinvention.

The compound used had the following structural formula: ##STR9##

Into a 500 ml three-necked flask equipped with mechanical agitationmeans, a gas inlet tube and a solid CO₂ condenser, the followingmaterials were introduced under a dry nitrogen blanket:

(i) 6.85 g (0.0283 M) of ##STR10##

(ii) 270 ml hexane (distilled over LiAlH₄).

The mixture was cooled to -40° C. and gaseous NH₃ was then introduced;the reaction was exothermic. During the introduction of the gaseous NH₃,the temperature was maintained at -28°C. The flow rate of the gaseousNH₃ was maintained at about 10 1/h of the gas; 12.5 1 of gaseous NH₃(0.558 mole) were introduced.

Ammonium chlorhydrate was formed during the experiment, which thickenedthe solution. Upon completion of the experiment, the NH₄ CL formed wasfiltered on sintered glass. The precipitate was washed several timeswith hexane (distilled over LiAlH₄). 2.9 g ammonium chlorhydrate wererecovered (3.05 g theoretical). The solution recovered was clear.

The solvents were evaporated under a pressure of 20 mm Hg; 4.8 g of alow viscosity, colorless oil were recovered.

The yield of the ammonolysis reaction was 91.3%

    ______________________________________                                        ---- Mn = 300        IP = 1                                                   ---- Mw = 300        ---- Mz = 400                                            ______________________________________                                    

The polymer was then pyrolyzed under argon. The TGA yield at 800° C. wasthen 6.0%.

EXAMPLES 2 to 8:

In accordance with the process of the invention, a co-ammonolysis wascarried out between BCl₃ and the difunctional compound used in thecomparative Example 1.

The procedure was identical to that of said comparative Example 1.

For these experiments, the molar ratio of: ##STR11## (T/D) was varied inthe respective starting mixtures.

The results are reported in the Table which follows, including thefollowing:

(1) the total concentration of the species T and D in the initialsolvent (conc.);

(2) the reaction temperature (T° C.);

(3) the yield of the co-ammonolysis reaction (yield %):

(4) the number and weight average molecular weights of the polymersobtained (Mn and Mw, respectively);

(5) the polydispersity index IP;

(6) the TGA yield at 800° C.

For each of Experiments 2 to 8, analysis evidenced that the resultingpolymers essentially consisted of a combination of structural units ofthe formula: ##STR12## and structural units of the formula: ##STR13##

EXAMPLE 9:

This example illustrates a co-ammonolysis according to the invention,between BCl₃ and ##STR14##

Into a 2 liter reactor under nitrogen, the following materials wereintroduced: 920 ml dry hexane, 47.0 g (0.401 mole) BCl₃ and 115.8 g(0.630 mole) of ##STR15## with the reactor previously being cooled to-37° C.

The ammonia was then introduced: 255 liters (11.38 moles).

The duration of the introduction of NH₃ was 5 h. After filtering,rinsing with hexane and the evaporation of the solvent, 29.1 g of ahighly viscous oil were recovered. The isolated yield was 31%.

    ______________________________________                                               TGA (850° C.)                                                                          31%                                                           ---- Mn         460                                                           ---- Mw         640                                                           IP              1.38                                                   ______________________________________                                    

Analysis evidenced that the polymer obtained essentially consisted of acombination of structural units of the formula: ##STR16## and structuralunits of the formula: ##STR17##

EXAMPLE 10:

This example illustrates a co-ammonolysis according to the invention,between BCl₃ and ##STR18##

Into a 2 liter reactor, under nitrogen, the following materials wereintroduced: 1.58 1 dry hexane, 78.6 g (0.67 mole) BCl₃ and 145.1 g (0.64mole) of ##STR19## with the reactor previously being cooled to 035°C.

The ammonia was then introduced: 310 liter (13.84 moles) NH₃.

The duration of the introduction of NH₃ was 6 h. After filtering,rinsing with hexane and the evaporation of the solvent, 90.2 g of aviscous and clear oil wree recovered. The isolated yield was 68.5%.

    ______________________________________                                        TGA (850° C.)   28.64%                                                 ---- Mn                620                                                    ---- Mw                1,100                                                  IP                     1.77                                                   ______________________________________                                    

Analysis evidenced that the resulting polymer essentially consisted of acombination of structural units of the formula: ##STR20## and structuralunits of the formula: ##STR21##

These results clearly show the advantage of the process according to theinvention in producing polymers based on boron and nitrogen having highmolecular weights and improved high temperature stability. Consequently,appreciably improved yields in boron nitride were obtained afterpyrolysis.

                                      TABLE                                       __________________________________________________________________________         Ratio                                                                     Example                                                                            ##STR22##  Conc. g/l                                                                          T °C.                                                                      Yield %                                                                            ##STR23##                                                                         ##STR24##                                                                          IP*                                                                              TGA % 800° C.               __________________________________________________________________________    2    23/77      84   -28 87.9 600  700 1.2                                                                              12                                  3    48/52      207  -35 66.8 700  900 1.3                                                                              18                                  4    53/47      70   -15 71.6 800 1300 1.6                                                                              22                                  5    63.5/36.5  78   -24 65.3 800 1200 1.5                                                                              17                                  6    65/35      90   -25 72.2 700 1100 1.5                                                                              24                                  7    75/25      89   -33 42.0 600  700 1.3                                                                              20                                  8    76/24      81   -22 33.8 900 1700 1.9                                                                              29                                  __________________________________________________________________________     *GPC Analysis: columns Shodex 3 (A 80 M) (l = 1.5 m) + one column at 802;     anhydrous CH.sub.2 Cl.sub.2 solvent (no alcohol)                         

While the invention has been described in terms of various preferredembodiments, the skilled artisan will appreciate that variousmodifications, substitutions, omissions, and changes may be made withoutdeparting from the spirit thereof. Accordingly, it is intended that thescope of the present invention be limited solely by the scope of thefollowing claims, including equivalents thereof.

What is claimed is:
 1. A crosslinked boron/nitrogen polymer, comprising (a) at least one recurring structural unit of the formula (I): ##STR25## and (b) at least one recurring structural unit of the formula (II): ##STR26## in which Y is N-R¹ R² and X is N-R³, wherein R¹, R² and R³, which are identical or different, are each a hydrogen atom, or a hydrocarbyl, organosilyl or hydrogenoorganosilyl radical.
 2. The boron/nitrogen polymer as defined by claim 1, wherein the molar ratio of the structural units (I) to the structural units (II) is at least one.
 3. The boron/nitrogen polymer as defined by claim 1, wherein R¹ and R² are each an alkyl, cycloalkyl, aryl, alkylaryl, arylalkyl, alkenyl or alkynyl radical.
 4. The boron/nitrogen polymer as defined by claim 1, wherein R¹ and R² are each an organosilyl radical.
 5. The boron/nitrogen polymer as defined by claim 4, wherein R¹ and R² are each a (triorgano)silyl radical.
 6. The boron/nitrogen polymer as defined by claim 5, wherein R¹ and R² are each a (trialkyl)silyl radical.
 7. The boron/nitrogen polymer as defined by claim 1, wherein R³ is a hydrogen atom or an alkyl radical.
 8. The boron/nitrogen polymer as defined by claim 7, wherein R³ is a hydrogen atom.
 9. The boron/nitrogen polymer as defined by claim 1, having a number average molecular weight ranging from 300 to 50,000.
 10. The boron/nitrogen polymer as defined by claim 9, having a number average molecular weight ranging from 500 to 5,000.
 11. The boron/nitrogen polymer as defined by claim 1, having a weight average molecular weight ranging from 600 to 100,000.
 12. The boron/nitrogen polymer as defined by claim 11, having a weight average molecular weight ranging from 1,000 to 10,000. 